Forging cracks, stress concentration, die design, and errors in the forging process.
| Identifier | ExFiles\Box 109\3\ scan0006 | |
| Date | 21th November 1939 guessed | |
| - 4 - is on the outside, the outside wall of the cavity is deeper than on the inside, as in an ordinary H sectioned arm, and if the draft angles are the same there will be a greater pressure on the larger area of the outside wall and a corresponding increase in leverage and stress in the corner adjacent to the deeper wall. This concentration of root stress is the cause of cracks in the die. It can, however, be reduced very considerably by having a true radius on the edge of the flange sections and not a flat and small radius on each corner. This modification has been carried out on several Griffon(Aero Engine) parts. Forging Cracks. When designing new work this feature should be very carefully considered as it has an important effect on grain flow and die life. As an illustration, some years ago we had many failures with H section handled 'C' spanners due to the flange cracking right through its thickness at the root and parallel with the web and "flash" line. This was caused by the very sharp (.025 to .030) root radius between flange and web. On altering this radius to .125 to .150 the trouble disappeared. The reason for the failure was due to the fact that the flange impression became filled with metal before the web was down to correct thickness and the subsequent pressure, coupled with a low forging temperature owing to the thin section and the root radius being so sharp, the metal would not flow around the impression and simply sheared through between the flanges, thus giving a crack. Thus, in planning a new forging after the essential bosses have been decided upon, the flash line must be determined and the joining up of the bosses should be as smooth as possible, and abrupt changes at right angles to the "flash" be avoided. The "Flash" line should preferably be straight, but can be set down in two or even three planes, but alternatives should be carefully considered before this course is taken. Balance in section thickness facilitates forging and reduces stress in dies. Errors in forging. Excluding faulty material, the most frequent error is in growth at right angles to the "flash" line, which is due to the dies not meeting during forging, whilst slight growth does occur on diameters due to wear of dies after long use. | ||
From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).